The present invention relates to a copying machine, printer, facsimile machine and image forming apparatus based on electrophotographic technology having the functions thereof, particularly to a color image forming apparatus having an intermediate transfer member, wherein a plurality of color toner images are superimposed on the intermediate transfer member.
In what is commonly known as an image forming apparatus based on electrophotographic technology using an intermediate transfer member, a toner image formed on an image carrier as an photoconductor is transferred onto the intermediate transfer member, and the toner image on the intermediate transfer member is transferred onto a transfer material (also called paper forms). In such a color image forming apparatus, the toner images sequentially formed on the image carrier and charged to have a predetermined polarity are superimposed and transferred onto an intermediate transfer member by static electricity. After that, the toner images on the intermediate transfer member are collectively transferred onto the transfer material.
The image forming apparatus using the aforementioned intermediate transfer member ensures that the toner image formed on the image carrier is superimposed on the intermediate transfer member, and therefore, is extensively employed in the color image forming apparatus for forming a color image on the transfer material. In this color image forming apparatus, the toner images of various colors formed on the image carrier are superimposed on the intermediate transfer member and are transferred thereon. Then the superimposed toner images are collectively transferred onto the transfer material by static electricity.
Since the amount of electric charge per toner particle is almost uniform, the electric potential of the toner layer on the intermediate transfer member is determined by the amount of toner deposited in a predetermined area. In the color image forming apparatus, the electric potential of electric charge in the portion where a plurality of the toners in different colors are superimposed in the toner images on the intermediate transfer member is greater than that in the portion where only the toner of one color is deposited. For example, when the toner image on the aforementioned intermediate transfer member has a solid portion and half-tone portion, the electric potential of the solid portion is greater than that of the half-tone portion.
After having passed through the primary transfer section wherein a toner image is transferred from the image carrier to the intermediate transfer member, the variation in the potential of the electric charge in the toner image may be produced depending on the environment.
As described above, if there is a big variation in the electric potential of the electric charge of the toner image on the intermediate transfer member, the portions with different transfer characteristics are present in one and the same toner image. If the portions with different transfer characteristics are to be transferred onto the transfer material under the same transfer conditions, various types of image failures tend to occur at the time of the secondary transfer from the intermediate transfer member to the transfer material.
In recent years, color printing technology has made a remarkable development in a copying machine, printer, facsimile machine and image forming apparatus having the functions thereof. As a result of adopting polymerized toner and small particle-sized toner, there has been a growing demand for higher image quality in the transfer process. Further, the speed in the image forming apparatus is getting higher and higher. To obtain a high quality image under this context, correction must be made to ensure that the electric potential of toner on the intermediate transfer member that varies with the frequency of primary transfer and environment is approximately uniform, thereby the secondary transfer performance is improved.
To solve aforementioned problems, Patent Document 1 proposes a structure having a pre-transfer charging device to charge the toner image after having been primarily transferred onto the intermediate transfer member before being secondarily transferred onto the transfer material, and which pre-transfer charging device disposes a conductive roller member arranged on the back of the intermediate transfer member opposed to the electrode of this charger and the charger whereby a counter electrode is formed. According to this method, a toner image primarily transferred onto the intermediate transfer member is charged by AC/DC corona discharging so that the amount of electric charge is approximately uniform.
The Patent Document 2 proposes a method of arranging a control section for controlling the charging conditions by the pre-secondary transfer charging device, in response to the traveling speed of the intermediate transfer member surface passing through the charging position where the pre-secondary transfer charging device charges.
[Patent Document 1] Official Gazette of Japanese Patent Tokkaihei 10-274892
[Patent Document 2] Official Gazette of Japanese Patent Tokkaihei 11-143255
According to the methods described in Patent Documents 1 and 2, the amount of electric charge of toner on the intermediate transfer member is uniformly set to a greater value. If the paper has a high resistance under low-humidity condition or during transfer onto the second face in the duplex copying mode, an image failure tends to be caused by the electric discharge due to the high electric potential of the paper. If the transfer voltage is reduced in order to avoid such an image failure, an insufficient transfer electric field occurs to the greater portion of the overall electric charge on the toner layer, and this will produce uneven density.
In the pre-transfer charging device described in Patent Documents 1 and 2 wherein a conductive roller member is arranged on the back of the intermediate transfer member opposed to the charger and a counter electrode is formed thereby, sufficient charging effects cannot be obtained easily if the speed of the image forming apparatus and the linear speed of the intermediate transfer member are increased. Further, the structure will become more complicated, for example, in order to control the charging condition in response to the linear speed of the intermediate transfer member.
In the meantime, if a flat counter electrode is pressed against the surface of the belt-shaped intermediate transfer member, the pressure at the central portion is inevitably reduced, with the result that the compression amount of the conductive elastic member is reduced. This will deteriorate the flatness of the belt-shaped intermediate transfer member. The distance between the grid of the scorotron electrode and the intermediate transfer member will be different according to the position. This will produce variations in the electric charge elimination performance of toner.